The present invention relates to safety performance of a driving circuit of the semiconductor laser, and more specifically relates to detection of abnormality in the semiconductor laser or in a photodetector utilized for power control of the laser, due to breakdown or degradation thereof. The present invention further relates to detection of lifetime of the semiconductor laser, and to forcible shutdown technology of the semiconductor laser in case of emergency.
Conventionally, the semiconductor laser is utilized as a component of a head of various information instruments such as information reading head of the compact disc player, information reading/writing head of the optical disc memory, bar code reader head provided in the POS terminal device, and print head of the laser beam printer.
The semiconductor laser is the most important component in these various information instruments. When an information instrument is inadvertently operated in spite of the semiconductor laser abnormality due to breakdown or degradation thereof, there may be caused a problem that information processing is not carried out correctly. Particularly, some type of the semiconductor laser emits an invisible light such as ultra-red ray, hence it would be difficult to recognize defect or degradation of the semiconductor laser. If the optical disc memory or the laser beam printer is driven in spite of failure or degradation of the semiconductor laser, recorded information may be inadvertently lost.
Further, these information instruments are generally provided with an automatic power control circuit including a photodetector operative to detect a laser beam intensity for maintaining the laser beam intensity at constant under the stable state. If the semiconductor laser is continuously driven in spite of the fact that some failure or degradation has occurred in the photodetector, excessive driving current will flow the semiconductor laser to produce abnormally high laser beam output which would dangerously damage human eye.
Further, when any failure occurs in the automatic laser power control mechanism, power control of the laser beam does not work, thereby causing danger of excessive emission of the laser beam. For example, if such excessive emission occurs in the optical disc memory, information recorded in an optical disc may be destroyed. Further, if such excessive emission occurs in a bar code reader provided in a POS terminal or in a so-called pointer which utilizes good directivity of the laser beam, intensive laser beam may inadvertently irradiate onto human eye to destroy the same. The conventional semiconductor laser driving circuit is not provided with safety measure for failure of the automatic laser power control mechanism.
Moreover, the semiconductor laser has drawback that it is gradually degraded along the passage of time so that its emission is reduced. For example, a laser diode has a varying emission intensity which will be reduced gradually, due to time-dependent degradation or aging, even though a constant forward current is applied thereto. As long as the automatic laser power control mechanism operates normally to follow the laser emission variation, the laser beam is controlled to maintain a constant level so that the forward driving current increases with aging of the laser diode. When the aging progresses into a certain degree where the automatic power control cannot follow, the laser beam intensity suddenly reduces to cause abnormal state, thereby resulting in unexpected disability of the divices.
Recently, semiconductor lasers of high performance are mass-produced at low cost. As described above, the semiconductor laser is utilized as a component of a head of various information instruments. The semiconductor laser can continuously emit a laser beam having good directivity. Therefore the laser beam is quite dangerous to destroy human eye, when an excessively intensive laser beam inadvertently penetrates into human eye due to occurrence of abnormality, or even when a weak laser beam continuously irradiates human eye for a long time. Such accident may happen, for example, in case that the semiconductor laser is utilized as a scanner in a bar code reader. Otherwise, in case that the semiconductor laser is utilized in a head of the optical disc memory, when a rotation speed of the optical disc is reduced or stopped due to some emergency, recorded data may be destroyed by intensive irradiation of laser beam. In view of the above noted possible emergency, the semiconductor laser must be immediately shut down when rotation is stopped or reduced in a scanning motor used for scanning a laser beam or a spindle motor used for driving a compact disc and an optical disc, or when controlling of a microcomputer becomes out of order in information instruments, or when a head is exposed during operation of information instruments. For this reason, there is provided an inhibiting circuit responsive to an inhibition signal for forcibly shutting down a semiconductor laser. However, the conventional semiconductor laser driving circuit is provided with a single of the inhibiting circuit. Therefore, when this inhibiting circuit is incidentally broken down, the semiconductor laser cannot be immediately turned off in case of emergency, thereby causing a problem that accident is not prevented effectively.